Bachmann Two-Truck Shay problem

Hello The loco just started to jerk and even not work at all. Used to run as smooth as can be. It's electrical caused I'm sure, as headlight also works only when it does move (jerks). Cleaned track and all wheels thoroughly. Still jerks. Inspected underside and whisked away anything that might be binding the mechanism. Still jerks. Any ideas are very much appreciated. Thanks in advance Brad